My prospective co-founder is a great person. The benefits to building an agency together is that we would have coverage in both markets. I want to minimize the chances of the relationship going south.
What aspect of the business would require the both people to be physically present in one location?
You can chat face to face online. Documents can be reviewed collaboratively just as if you were sitting at the same table.
A 3-hour time zone difference is negligible, since the overlap between your working day and time awake is so large.
Apart from teaming up to move pianos or catching fewer colds from one another, what would be missing?
Lay the foundation for a stable and long lasting partnership by writing out an operating agreement with the shares of ownership distributed to the different parties and a very detailed description of what role each party will play in the new venture. The last thing you want to do is leave anything open for interpretation.